Technical Field
The present invention relates to a black ink composition containing carbon black, an ink set including the composition, and an image forming method.
Description of the Related Art
Inkjet recording methods allow recording of high-quality images on various recording media by ejecting droplets of inks from a number of nozzles provided at an inkjet head. Therefore, inkjet recording methods are widely used.
Pigments are widely used as a coloring agent, which is a component of an ink. In inks, pigments are used in a state of being dispersed in a medium such as water. When pigments are used in the dispersed state, important factors include the diameter of the dispersed particles, stability after dispersing, size uniformity, ejectability from ejection nozzles, and image density. Various studies are carried out on techniques for improving such factors.
Further, in some cases, recording on plain sheet provides only insufficient quality in terms of rubbing resistance of the formed image (fixability), resolution, and the like, as well as image density. There is a tendency for the insufficiency in quality to become more conspicuous when the inkjet recording speed is increased. Specifically, compatibility with high-speed recording in a single-pass manner rather than a shuttle-scan manner is requested in terms of the rubbing resistance and resolution of the formed image. In the single-pass recording, recording can be carried out by a single head operation.
Meanwhile, in addition to the image density, generation of unevenness in image has to be considered from the viewpoint of image quality because unevenness in image, such as streak-shaped unevenness, may be generated when an image is recorded on plain paper or the like. The generation of unevenness is significant when high-speed inkjet recording is carried out. Specifically, streak-shaped unevenness is more likely to be generated in an image when the image is recorded at high speeds in a single-pass manner, which enables recording with one operation of a head, as compared to a shuttle-scan manner. The unevenness is particularly remarkable when an image of an intermediate tone of black (i.e., gray tone) is formed.
When high-speed recording is carried out in a single-pass manner, an inkjet head having a width that is equal to the width of a recording medium is used. However, in such a case, unevenness in hue tends to be generated in the width direction owing to a variation in ejection amounts of ink along the width direction. The unevenness in hue is particularly significant when an image of intermediate tone of black (i.e., gray tone), which is achromatic, is formed.
Among pigments used in inks for inkjet recording, carbon black is widely used as a black pigment. A technique in which a pigment other than carbon black, such as a cyan pigment, is used together with carbon black (hereinafter abbreviated to CB) is also known.
Specifically, a pigment dispersion aqueous recording liquid is disclosed in, for example, Japanese Patent Application Laid-Open (JP-A) No. 2004-285344. This pigment dispersion aqueous recording liquid contains carbon black (CB) and a pigment other than CB, such as a cyan pigment, together with a urethane resin. Further, an aqueous ink for inkjet recording that contains polymer particles containing carbon black is disclosed in, for example, JP-A No. 2009-144007.
Regarding an ink containing carbon black (CB), a neutral black ink or black ink set which contains carbon black as well as a cyan pigment and a magenta pigment is disclosed (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2005-213505, or JP-A No. 2003-55592), which enables improvements in tone of an yellowish or brownish image, and provides excellent color reproducibility.
Furthermore, a black ink is disclosed (see, for example, JP-A No. 2009-132766), which contains a dispersion of surface-treated black pigments capable of being dispersed in water even in the absence of a dispersant, a magenta pigment-containing polymer particle dispersion, a cyan pigment-containing polymer particle dispersion, a yellow pigment-containing polymer particle dispersion, and resin emulsion. It is disclosed that the black ink is excellent in glossiness of image, ejection property, and the like.
On the other hand, a pigment, being hard powder, has a problem in that the pigment may easily damage a nozzle surface when an ink is wiped off for maintenance of a nozzle for ejecting the ink. In particular, when carbon black, from among various pigments, is contained in an ink, the ink tends to damage a nozzle surface as compared with a case in which a color ink containing an organic pigment is used.